1. Field of the Invention
The present invention relates to predistorters (PDs) that use a predistortion method to compensate for nonlinear distortion generated in an amplifier and, in particular, to a predistorter in which predistortion coefficients are converged efficiently.
2. Description of the Related Art
Power amplifiers generally have linear input-output characteristics at lower input levels, while having nonlinear characteristics over a certain input level, and further provide saturated output power. Power amplifiers are normally used at operating points close to their own saturation points to achieve higher power efficiency, which causes nonlinear distortion due to the nonlinearity thereof. Such nonlinear distortion results in leakage of unnecessary signal components within and outside of the desired signal bands (into adjacent channels).
In general, amplifiers provide a complex output signal y(t) as expressed in Formula 1, where “t” is time and z(t) is a complex input signal of each amplifier.
In Formula 1, “Gain” is a real number representing the gain of the amplifier. Dn is a complex coefficient for an n-th order nonlinear distortion coefficient generated in the amplifier.
Formula 1 is a power series expansion of the input-output characteristics of the amplifier, where Gain·z(t), the first term on the right-hand side, is a linear component (desired wave), while the second and following terms on the right-hand side are nonlinear components (spurious waves).Formula 1y(t)=Gain·z(t)+D3|z(t)|2z(t)+D5|z(t)|4z(t)+D7|z(t)|6z(t)+  (Formula 1)
In Formula 1, only odd-order terms, i.e., third, fifth, seventh . . . order terms are included for the reason that observing the frequency spectrum of the output signal from the amplifier, odd-order distortion spectra appear near the spectrum of the linear component Gain·z(t). On the other hand, even-ordered distortion, which appears as higher harmonic waves of two times or more of differential frequency components generated within the baseband, can be attenuated easily through a band-limiting filter (or band-pass filter) or the like.
In particular, base station apparatuses, which require higher transmission power, are subject to severe regulations on nonlinear distortion, such as ACLR (Adjacent Channel Leakage power Ratio), spurious standards, and spectrum emission masks. Thus, how to reduce nonlinear distortion has been a big problem.
One way to compensate for such nonlinear distortion in power amplifiers is to use a predistortion method. Predistortion methods can compensate for distortion generated in a power amplifier by providing reverse characteristics of AM-AM conversion and/or AM-PM conversion as nonlinear characteristics of the power amplifier preliminarily applied to the input signal of the amplifier.
FIG. 1 shows an exemplary configuration of an amplifier with a predistorter that uses a predistortion method to compensate for distortion in the power amplifier.
FIG. 2 shows an exemplary configuration of a predistortion executing section 13.
It is noted that FIGS. 1 and 2 will be referred to below in embodiments of the present invention, and here referred to only for the sake of descriptive convenience, not intended to limit the present invention unnecessarily.
Here, adaptive update algorithms for the distortion compensation table 12 include a waveform comparison method and a perturbation method in which out-of-band distortion power is a function of error.
In the waveform comparison method, an error signal is calculated in the control section 14 based on a feedback signal acquired from the A/D converter 7 (output signal from the power amplifier section 4 containing distortion) and an input signal (input from the input side into the control section 14), and an algorithm such as LMS (Least Mean Square error) is used to converge the contents of the distortion compensation table 12.
In the perturbation method, out-of-band power calculated by Fourier transforming feedback signals is used as an evaluation function, and coefficients of the predistorter 1 are changed and the one with the smallest evaluation function is selected to converge the contents of the distortion compensation table 12.
The foregoing methods have their respective features. The waveform comparison method requires considerably high accuracy for adjustment of the delay time between and the amplitude of input and output signals, and therefore requires complicated processing. On the other hand, the perturbation method, which uses only feedback signals, can be achieved with relatively simple processing, but takes a long time for convergence.
It is noted that FIG. 7 shows an exemplary configuration of a control section 101 in the case where a waveform comparison method is used as an adaptive algorithm for the coefficients of the predistorter, which will be described below in detail in an embodiment (Embodiment 4).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-112151
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-101908
As for such conventional predistorters as mentioned above, there is still room for development in efficiently converging predistortion coefficients (e.g. the contents of the distortion compensation table 12) and there has been desired a further improvement in efficiency.
As a specific example, memoryless predistorters, in which coefficients in predistorters of different orders affect each other, suffer from a problem of taking a long time for adaptive convergence.
Similarly, memory effect compensation predistorters (memory effect predistorters), in which coefficients in predistorters of different orders affect each other, also suffer from a problem of taking a long time for adaptive convergence.
Combined use of such two kinds of predistorters also suffers from a problem of taking a long time for adaptive convergence because the predistorters affect each other even if the coefficients in the respective predistorters may be independent of each other.
Further, as for such conventional predistorter configurations as mentioned above (e.g. as shown in FIG. 7), there is still room for development in converging predistortion coefficients (e.g. the contents of the distortion compensation table 12) using an adaptive algorithm and there has been desired a further improvement inefficiency.